fix: per-workstream SSE fan-out — multiple consumers no longer steal … (#38)

* fix: per-workstream SSE fan-out — multiple consumers no longer steal each other's tokens

After 4d665a5 removed the single-consumer SSE lock, the shared
_event_queue let concurrent consumers (browser, bridge, console proxy)
race on Queue.get(), each receiving ~1/N of content tokens and producing
garbled streaming text.

Replace the single queue with per-client fan-out: each SSE connection
registers its own bounded queue (maxsize=500) on WebUI._listeners, and
_enqueue() copies every event to all registered queues. On eviction or
close, a ws_closed sentinel is injected so SSE generators exit promptly.

* fix: address CI failures and Copilot review feedback

- Handle ws_closed sentinel in events_sse generator (break on close)
- Guarantee sentinel delivery by evicting one item when queue is full
- Clear listeners list after injecting sentinels on cleanup
- Fix test_slow_consumer to fill only slow queue directly
- Fix ruff SIM117 (nested with), unused import, mypy unused-ignore

docs/diagrams/png/03-core-engine-classes.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:c53ddce800c59f9432d7a016c7d66282a449555d452b7fe9dd393f4282f08c46
-size 554721
+oid sha256:760c37e67736588dadee21d500419a48e9fc50f8bdc5667e686c580022bd40e2
+size 554869